In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, photovoltaic (PV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art various materials have been proposed for use as organic semiconductors in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processability, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement.
One aim of the present invention is to provide new organic semiconducting materials for use in electronic devices, which have advantageous properties, in particular good processability, high charge-carrier mobility, high on/off ratio, high oxidative stability and long lifetime in electronic devices. Another aim is to extend the pool of semiconducting materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It has been found that these aims can be achieved by providing semiconducting materials as claimed in the present invention. These materials are based on polymers comprising one or more cis- or trans-indenofluorene units of the following formulae, or derivatives thereof
(wherein R1-4 denote aromatic or aliphatic hydrocarbyl groups, and two adjacent groups R1,2 or R3,4 may also form a spiro group),and further comprising one or more moieties having hole or electron transport properties, like for example triarylamine groups. In particular, it has been found that such polymers are suitable for use as semiconducting materials in electronic devices like transistors, and as charge transport layer or interlayer in polymer light emitting diodes (PLEDs), as they have good processability and at the same time show a surprisingly high charge carrier mobility and high oxidative stability.
WO 2004/041901 describes polymers comprising aryl-substituted indenofluorenes and further units like triarylamine or heteroaryl moieties, and their use in OLED or OFET devices, but does not disclose polymers as claimed in the present invention. WO 2005/024971 shows a specific copolymer comprising 30% 11,11,12,12-tetraoctyl-indenofluorene, but does not disclose polymers as claimed in the present invention.
Triarylamines have good hole transport ability. However, they have only moderate solubility in many organic solvents, which does negatively affect their processability in a film-forming process and leads to films with moderate uniformity. Indenofluorenes, on the other hand, are soluble in conventional organic solvents and thus show good processability, enabling the formation of films with high uniformity. However, they have been reported mainly to be efficient electron transporting and light-emitting moieties. It was therefore surprising that a polymer according to the present invention, wherein indenofluorene units are combined with hole transporting moieties like triarylamines, shows high charge carrier mobility and enables the preparation of electronic devices like transistors with high on/off ratios.